The present invention relates to a network interface apparatus that can transfer data frames according to network standards, particularly to Ethernet standards, while monitoring the collision of data frames. This invention also relates to a data transmitting apparatus incorporating the network interface apparatus and a data transmitting system comprising data transmitting apparatuses of this type.
Network technology is applied to process information (in data-processing systems) or to control systems and the like (in monitoring/controlling systems). LAN (Local Area Network) is now widely utilized in information-processing systems. Particularly, Ethernet-based LAN and IEEE 802.2-based LAN (hereinafter collectively referred to as xe2x80x9cEthernetxe2x80x9d) have remarkably advanced. Ethernet is now used not only in information-processing systems, but also in monitoring/controlling systems. This owes to the increase of data transmitting speed and the performance enhancement of communications apparatus, both attained by technological advance, and also to the reduction in the price of standardized apparatuses, achieved by mass-production.
Various Ethernet LANs are actually installed. One of them is a network having a 10 Mps bus-type transmission path. Another is a network which is composed of the network and devices and repeaters or switching hub devices, and which therefore has a star-type transmission path. Each of these Ethernet LANs can be easily provided by using, in appropriate combination, Ethernet devices that have various transmission paths, such as a 10 Mbps path, a 100 Mbps path and a 1 GMbps path.
In Ethernet, which is used at present and will be used in the future, too, the CSMA/CD (Carrier Sense Multiple Access/Collision Detection) system is employed as the transfer control method. As long as the CSMA/CD system is used in Ethernet, Ethernet has the following disadvantage in connection with the timing of transmitting frames. No matter how much the speed of transferring data increases thanks to technical advances, it is impossible to predict accurately how long each transmitting station (i.e., data-transferring station) must wait before reliably transmitting station frames through the transmission path.
In the CSMA/CD system, each transmitting station monitors carriers on the common transmission paths. If the common transmission paths is vacant for a predetermined time, the transmitting station starts transmitting frames. If the common transmission path is not vacant, that is, if there are carriers on the paths the transmitting station does not transmit frames until the transmission path becomes vacant. Even after starting the transmission of frames, the transmitting station monitors the transmission path to determine if any frame it has transmitted is colliding with the frame transmitted from any other transmitting station. If the transmitting station detects a collision, it stops transmitting frames and tries to transmit the frames again upon laps of a prescribed time.
The more frequently the transmitting stations use the transmission path, the higher the possibility that the frames collide with one another in the transmission path. If any frame collides with another frame in the transmission path, the transmitting stations will hardly transmit frames reliably. In other words, each transmitting station cannot transmit frames by any time specified without fail. It cannot be predicted how long the transmitting station must wait before transmitting frames. Hence, Ethernet has but limited use in real-time control of an object or a process, which is achieved by exchanging information within a predetermined time.
As mentioned above, Ethernet is now used in monitoring/controlling systems, as well. Sufficient response speeds are secured by re-designing the system and operating the system in an adjusted scheme. Ethernet can therefore transmit information at high speed. Nonetheless, the traffic of information is minimized and the transmitting stations constituting the transmission system are reduced in numbers. Therefore, the monitoring/controlling systems need not operate at its maximum capacity, and does operate readily and reliably.
To exchange information reliably within a predetermined time, thus in real time fashion, a system that realizes token passing is proposed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 4-39819. This system known as xe2x80x9cimplicit token passxe2x80x9d performs implicit token passing, not effecting such explicit exchange of token frames as effected in a token passing system represented by the IEEE 802.2-based.
The data transmitting apparatus (implicit token path system) disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-39819 can use the Ethernet transmission control LSI. In this data transmitting apparatus, a special signal pattern is added to the frame format specified by Ethernet, whereby the timing of staring a token path is defined by the sync signal transmitted from a specific synchronous transmitting station. Each transmitting station can transmit frames, one after another, after the sync signal. This inhibits collision of frames, which inevitably occurs in the CSMA/CD system.
The data transmitting apparatus can utilize the LSI, connectors, cables, software resources and the like, which constitute Ethernet. The train of signals propagating through the transmission path has a frame format that differs from the one defined by Ethernet.
It is therefore impossible to incorporate a network of the implicit token path system into any Ethernet-based, data transmitting system, i.e., a network system that comprises repeaters, hub devices, Ethernet devices (e.g., switching hub devices) and Ethernet transmitting stations. The network of the implicit token path system can indeed use Ethernet-based devices, but it must be a data transmitting system different from and independent of Ethernet.
The object of the present invention is to provide a data transmitting apparatus which is to be connected to a communication system comprising devices (e.g., Ethernet-based ones) using protocol designed to prevent collision of frames, and which accomplishes communication among the devices, enabling transmitting stations incorporated in a network (e.g., Ethernet) to exchange information. The other object of the invention is to provide a data transmitting apparatus, a network interface apparatus and a data transmitting system which can transmit frames within a prescribed time, thus in real time fashion, not influenced by the data communication among the transmitting stations provided in a communication system.
According to the first aspect of the invention, there is provided a data transmitting apparatus corresponding to one of a plurality of data transmitting apparatuses constituting a specific network. The data transmitting apparatus comprises: connection means connected to a communication system including the specific network, the communication system being able to include a terminal which operates in accordance with a protocol to transmit no signals through a data transmission path while another signal is transmitted through the data transmission path, to stop transmitting a signal if the signal collides with other signals in the data transmission path, and to transmit the signal again after a period for preventing collision of signals has elapsed;
signal receiving means for receiving signals transmitted from the terminal or any one of the other data transmitting apparatus, the signals transmitted through the data transmitting path and having a data format that accords with the protocol; transmission right acquisition means for determining, at a specific timing, whether the data transmitting apparatus has acquired transmission right that should be given cyclically in the specific network, from information representing an order in which the transmitting apparatuses of the specific network should acquire the transmission right; and signal transmitting means for transmitting signals to the data transmission path in the data format that accords with the protocol, for transmitting signals, no matter whether signals exist in the data transmitting path, when the transmission right acquisition means determines that the data transmitting apparatus has acquired the transmission right, and for continuously transmitting signals even before the period for preventing collision of signals has elapsed, even when collision of signals occurs in the data transmission path, in order to prevent the signals to collide again in the data transmission path.
As long as the data transmitting apparatus holds the transmission right, it transmits signals, even if other signals exist in the data transmission path, thus causing the signals to collide with the other signals in the data transmission path. Further, the data transmitting apparatus continues to transmit signals without adjusting the transmission timing even when the signals collide with the other signals. Therefore, the apparatus can transmit data reliably. Since the transmission right is cyclically shifted from one data transmitting apparatus to another, each data transmitting apparatus can transmit frames within a predetermined time in the network, even if the apparatus is connected to an Ethernet terminal of an Ethernet-protocol system. The data transmitting apparatus can therefore operate in real-time fashion.
According to the second aspect of this invention, there is provided a network interface apparatus for use in one of a plurality of data transmitting apparatuses constituting a network. The network interface apparatus comprises: connection means connected to a communication system including the specific network, the communication system being able to include a terminal which operates in accordance with a protocol to transmit no signals through a data transmission path while another signal is transmitted through the data transmission path, to stop transmitting a signal if the signal collides with other signals in the data transmission path, and to transmit the signal again after a period for preventing collision of signals has elapsed; signal receiving means for receiving signals transmitted from the terminal or any one of the other data transmitting apparatus, the signals transmitted through the data transmitting path and having a data format that accords with the protocol; transmission right acquisition means for determining, at a specific timing, whether the data transmitting apparatus has acquired transmission right that should be given cyclically in the specific network, from information representing an order in which the transmitting apparatuses of the specific network should acquire the transmission right; and signal transmitting means for transmitting signals to the data transmission path in the data format that accords with the protocol, for transmitting signals, no matter whether signals exist in the data transmitting path, when the transmission right acquisition means determines that the data transmitting apparatus has acquired the transmission right, and for continuously transmitting signals even before the period for preventing collision of signals has elapsed, even when collision of signals occurs in the data transmission path, in order to prevent the signals to collide again in the data transmission path.
According to the third aspect of the present invention, there is provided a data transmitting system comprising. The data transmitting system comprises: a data transmission path; at least one terminal which operates in accordance with a protocol to transmit no signals while signals are transmitted through a data transmission path, to stop transmitting signals if the signals transmitted collide with other signals in the data transmission path, and to transmit the signals again after a period for preventing collision of signals has elapsed: and a plurality of data transmitting apparatuses, each comprising signal receiving means for receiving signals transmitted from the data transmitting path in a data format that accords with the protocol, transmission right acquisition means for determining, at a specific timing, whether the data transmitting apparatus has acquired transmission right that should be given cyclically, from information representing an order in which the data transmitting apparatus should acquire the transmission right, and signal transmitting means for transmitting signals to the data transmission path in the data format that accords with the protocol, for transmitting signals, no matter whether signals exist in the data transmitting path, when the transmission right acquisition means determines that the data transmitting apparatus has acquired the transmission right, and for continuously transmitting signals even before the period for preventing collision of signals has elapsed, even when collision of signals occurs in the data transmission path, in order to prevent the signals to collide again in the data transmission path.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.